Technologies available for the rate quantification of gas(es) emitted from a point source such as a smokestack or a leaking pipeline are numerous and well understood. These techniques include use of rotameters or hot-wire anemometers to measure the velocity of gasses escaping from a hole of known size. Also included are more high tech devices like the Hi-Flow sampler which is an instrument that combines a hydrocarbon detector with a flowmeter and vacuum system.
Measurement of emission flux from larger, more complex non-point sources of emissions may be accomplished by numerical calculations within the facility (for example, estimating based on amount of material lost), by direct point-sampling of the gas plume downwind of the source, or (most recently) by use of path integrated optical remote sensing. Non-point-source emissions of gasses like methane and other volatile organic carbons (VOCs) are numerous and variable and include refineries, industrial complexes, sewage systems, tank farms, landfills, agricultural sites, coal mines, oil and gas exploration and production sites and pipeline networks. Measuring the emissions from these types of sites is made more difficult by the fact that exact sources of emission are not always known.
There are several optical technologies available to quantify the gasses emitted from non-point sources. These optical technologies include Open-Path Fourier Transform InfraRed Spectroscopy (OP-FTIR), Ultra-Violet Differential Optical Absorption Spectroscopy (UV-DOAS), Tunable Diode Laser Spectroscopy (TDLAS), and Path Integrated Differential Absorption Lidar (PI-DIAL). These optical remote sensing technologies are all ground based, active optical instruments which pass light through a plume of gas and measure a path-integrated concentration of gas in the plume by detecting changes in light passing through the gas plume. Further, these technologies rely on one or more retro-reflectors or separate light sources and detectors on either side of the gas plume.
Measurement approaches include horizontal plume mapping, vertical plume mapping and one-dimensional “fenceline” mapping downwind of a plume. Another measurement technique is Solar Occultation Flux (SOF) which uses IR and the sun as an optical source from a fixed or moving ground-based platform. Combining path-integrated gas measurements with measurements of wind speed and direction allows calculation of the amount of gas emitted from a location over time (flux).